1. Field of the invention
The present invention relates to a fringe field switching mode LCD, and more particularly to a fringe field switching mode LCD adapted to adjust the angle between the liquid crystal director and the composite electric field in the fringe field switching mode to improve the gray-to-gray response time.
2. Description of the Prior Art
A fringe field switching mode (hereinafter, referred to as FFS-mode) LCD has been proposed to improve the low aperture ratio and transmittance of in-plane switching mode (hereinafter, referred to as IPS-mode) LCDs, as disclosed in Korean Registered Patent No. 10-0341123.
FIG. 1 shows a conventional FFS-mode LCD.
Referring to the drawing, the FFS-mode LCD 10 includes a lower glass substrate 11; a counter electrode 12 and a pixel electrode 13 formed on the lower glass substrate 10 using ITO with a gate insulator 14 interposed between them; an upper glass substrate 15; and a color filter 16 formed on the upper glass substrate 15 with a liquid crystal layer 17 interposed between them, which is composed of a number of liquid crystal molecules. The lower and upper glass substrates 11 and 15 are bonded to each other.
The FFS-mode LCD 10 constructed as above has higher aperture ratio and transmittance than IPS-mode LCDs, because the counter electrode 12 and the pixel electrode 13 are made of a transparent conductor. In addition, the spacing between the counter electrode 12 and the pixel electrode 13 is smaller than that between the upper and lower glass substrates 11 and 15 to establish a fringe field between the counter electrode 12 and the pixel electrode 13. As a result, even the liquid crystal molecules existing on top of the electrode 13 are operated and improve the transmittance.
The transmittance control in the FFS-mode LCD is performed by controlling the twist angle of liquid crystals. For a high transmittance, the liquid crystals are rubbed while director being about 83° relative to the electric field direction, as shown in FIG. 2.
The rising response time of the liquid crystals is determined by the following equation:τon=r1/{N−(π2/d2)/K2  (Equation 1)
In Equation 1, the dielectric torque N in the denominator determines the rising response time and the remaining values are constants. The dielectric torque N is expressed as follows:N=|Δε(n·Ε)n×Ε|=ΔεΕ02 sin 2Ψ  (Equation 2)
It is clear from both equations that, under the condition of the same electric field intensity, the fastest rising response time is obtained when sin 2Ψ=1(Ψ=45°), particularly, when the angle between the alignment direction of the liquid crystal molecules and the direction of the initial electric field is 45°.
FIG. 3 shows the angle between the electric field and the average director of the liquid crystal molecules corresponding to each gray level, when the gray level varies in the FFS-mode LCD. It is clear from FIG. 3 that, as the gray level increases and the angle approaches 45°, the rising gray-to-gray response time becomes faster and, as the angle goes beneath 45° and becomes smaller, the rising gray-to-tray response time becomes slower again.